This relates to semiconductor devices and processes, and more specifically to the structure and fabrication of hermetic plastic packages for microelectromechanical system (MEMS) devices.
The wide variety of products collectively called micro-electromechanical system (MEMS) devices are small, lightweight devices on the micrometer to millimeter scale, which may have mechanically moving parts and often movable electrical power supplies and controls, or they may have parts sensitive to thermal, acoustic, or optical energy. MEMS devices have been developed to sense mechanical, thermal, chemical, radiant, magnetic, and biological quantities and inputs, and produce signals as outputs. Because of the moving and sensitive parts, MEMS devices have a need for physical and atmospheric protection. Therefore, MEMS devices are supported on a substrate and surrounded by a housing or package, to shield the MEMS components against ambient and electrical disturbances, and against stress.
A MEMS device integrates mechanical elements, sensors, actuators, and electronics on a common substrate. The manufacturing approach for a MEMS device aims at using batch fabrication techniques similar to those used for other microelectronics devices. Such devices can thus benefit from mass production and minimized material consumption to lower the manufacturing cost, while trying to exploit the well-controlled integrated circuit technology.
Example MEMS devices include mechanical sensors, both pressure sensors including microphone membranes, and inertial sensors such as accelerometers coupled with the integrated electronic circuit of the chip. The mechanically moving parts of a MEMS devices are fabricated together with the sensors and actuators in the process flow of the electronic integrated circuit (IC) on a semiconductor chip. The mechanically moving parts may be produced by an undercutting etch at some step during the IC fabrication. Examples of specific bulk micromachining processes employed in MEMS sensor production to create the movable elements and the cavities for their movements are anisotropic wet etching and deep reactive ion etching.
MEMS device packages do not generally have to be hermetic, i.e., impermeable to water molecules. Consequently, typical MEMS devices may use sealants made of polymeric compounds. Some MEMS devices do, however, require hermetic packages. Example of devices requiring hermetic packages include digital micromirror devices (DMDs), such as the DLP® DMD devices available from Texas Instruments. A typical DMD includes an array of individually addressable light modulating pixel element micromirrors, the reflectors of each of which are selectively positioned to reflect or not to reflect light to a desired site.
Conventional hermetic packaging of MEMS devices usually involves a packaging process that departs from the processes normally used for non-MEMS device packaging. MEMS hermetic packaging is expensive not only because the package often includes a ceramic material, or a metallic or glass lid, but also because the package must be configured to avoid contact with moving and other sensitive parts of the MEMS device and to further allow a controlled or reduced atmosphere inside the package. The high package cost is, however, in conflict with market requirements for many applications of MEMS devices, which put a premium at low device cost and, therefore, low package cost. Further, the conventional fabrication of hermetic MEMS packages also encounters many technical challenges, such as those caused by potentially high temperatures in connection with welding of a hermetic lid to the package base. As an example, a recently proposed package with a sealing process using a glass core involves temperatures considerably above 450° C., typically between 525 and 625° C. dependent on the sealing glass selected. These temperature ranges are a risk for the reliability of silicon integrated circuits and for proper functioning of many MEMS device components.
There is a need for low cost hermetic packaging of MEMS devices.